The present invention relates generally to clutter filters, and more particularly to an improved clutter filter for rejecting multiple clutter types in a moving target indication (MTI) radar.
A radar return usually contains both the target signal and a clutter signal. The clutter signal arises from reflections from stationary background objects which usually are much stronger than the target signal. This unwanted clutter, however, can be discriminated against by use of a clutter filter. This type of filter operates on the principle that a moving target has a Doppler frequency shift, but the stationary clutter signal has none. There are two ways to design such a clutter filter. The first way is to design a filter which divides the Doppler frequency space into two regions--a stop-band region and a pass-band region. By properly choosing the width and location of the filter's stop-band regions, one can effectively eliminate the clutter noise to an acceptable level. This type of filter is generally referred to as an MTI canceler. The second type is a filter which divides the Doppler frequency space into many regions. Each filter output responds to one of these narrow bands. This type of filter is generally referred to as a Doppler filter and is formed by a FFT (Fast Fourier Transform) network. This filter not only can be used to discriminate against the unwanted clutter, it can also be used to resolve the target Doppler frequency and provide improvement against noise. However, in many practical cases, neither of these two types of filter is adequate. For example, if the radar returns contain moving clutter of different types, each having a different Doppler frequency characteristic, it is evident that a simple MTI canceler would not be enough to achieve the required clutter rejection level. Theoretically, this type of clutter may be handled by a multiple-band (FFT) filter. However, due to the limitation of sidelobe level, a Doppler filter usually has a low improvement factor. Sometimes, this improvement factor may not be enough to reject a strong clutter signal. To alleviate this problem, it has been proposed that a multiple-stage filter connected in cascade be used. For example, the paper "Technical Note 1972-14", MIT Lincoln Lab., Oct. 1972, pp 1-16, by R. McAulay discloses that one may use a MTI canceler followed by a Doppler filter. Or, one may use several stages of MTI cancelers. Each stage is used for filtering a particular type of clutter.
In a radar system, the number of pulses to be used for coherent filtering is always limited due to the data rate, the requirement of frequency agility and other considerations. However, in general, the proposed multiple-stage filter requires many radar pulses because the total number of pulses required is approximately equal to the sum of the number of pulses required in each stage.